Frequency modulated altimeter or distance indicator



June 10, 1947.

ILC

w. L. CARLSON ET Al.

FREQUENCY MODULATED ALTIMETER OR DISTANCE INDICATOR Filed Nov. 21, 1942 lax/mme l I i mT/Mf meca/7 nabllhlwm nventorS Patented June 10, 1947 UNHTED STATES OFFICE FREQUENCY MODULATED ALTIIVIETER R DISTANCE INDICATOR f Application November 21, 1942, Serial No. 466,402

Our invention relates to radio systems such as radio altimeters for indicating distance or height and will be described particularly with respect to systems wherein the radiated wave is frequency modulated.

Systems of this character are described in Bentley Patent 2,011,392 and Espenschied Patent 2,045,071. In these systems, the radiated frequency modulated wave is reflected from the earths surface or other object and the reflected wave is received in a heterodyne receiver located in the vicinity of the transmitter. The heterodyning or mixing signal for the receiver is obtained directly from the transmitter whereby the receiver output includes a signal of beat frequency which frequency is a function of the time required for the radiated signal to reach the reflecting object and return to the receiver..

It has been found that one of the things that limits the full use of the sensitivity of the receiver is the interferingr signal resulting from the amplitude modulation normally present on the mixing signal that is applied to the heterodyne detector. When the optimum mixing voltage, i. e., optimum with respect to noise ratio, is introduced on the heterodyne detector, the amplitude modulated signal in the output of the receiver is still ten to fifty times the noise level when the transmitter signal is swept through a twenty megacycle bandwidth, for example.

It might be expected that there would be no amplitude modulation on the mixing signal. This would be true if it were not for the difficulty of transmitting a wide frequency band signal over a line with uniform attenuation at all frequencies. In actual practice there are reections on the line to the transmitter antenna and on the line between the transmitter and the receiver because of practically unavoidable imperfect line terminations, for example.

While amplitude modulation on the mixing signal causes the greater part of the above mentioned interference, there also may be some interference, usually almost negligible, caused by amplitude modulation on the received reflected signal. The latter modulation is introduced because of such factors as imperfect termination of the line from the transmitter to the transmitter antenna and possibly because of frequency selective reflection of the signal.

An object of our invention is to provide an improved method of and means for reducing the interfering amplitude modulation signal appearing in the output of a receiver in a frequency modulated distance determining system. A fur- 12 Claims. (Cl. Z50-1.68)

A2 ther object of the invention is to provide an improved system of the above-described type,

In practicing a preferred embodiment of the invention, the transmitter radiates to the earth or other reflecting surface a frequency modulated carrier wave having a carrier frequency twice that of the frequency modulated carrier Wave which is supplied directly, i. e., without reflection, to the receiver asa mixing signal. The radiated carrier wave, after travelling from the transmitter to the reecting surface and back to the receiver, is also applied to the receiver.

The detector in the receiver may comprise two similar diodes connected in parallel opposition. This results in a detector having a symmetrical characteristic curve of volts input vs. current output, Any amplitude modulation that may be present on either of the two applied carrier waves is balanced out since the amplitude modulating outputs from the two diodes are of opposite polarity and feed into a common output resistor. Likewise, any interfering vor jamming signal may be balanced out.

The operation of the detector in producing a beat frequency by the use of a double frequency signal is based upon the method described in Carlson Patent 1,734,894', issued November 5, 1929.

The invention will `be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram of a radio altimeter designed in accordance with one embodiment of our invention, Figures 2 and 2av are graphs which are referred to in explaining the operation of the apparatus, and Figure 3 is a diagram illustrating the use of a different detector in the receiver. In the several gures like parts are indicated Iby similar reference characters.

Referring to Fig, 1, the altimeter comprises a frequency modulated radio-frequency oscillator 9 which has its output doubled in frequency by a frequency doubler I0. The output of the doubler I`0 is supplied toa dipole l I whereby a frequency modulated signal is radiated toward the earth. Such a signal is produced by sweeping the frequency of the carrier wave of the oscillator 9 through a frequency band periodically, i. e., by cyclically frequency modulating the carrier wave. This may be done by means of a suitable frequency modulator l2, which may be, for example, a motor driven capacitor or a vibrating capacitor (not shown) for varying the tuning of the transmitter oscillator 9.

The receiver includes a detector l5 comprising two diodes I6 and l1 which are connected in parallel opposition. The reflected signal is picked up by a dipole I8 and applied to the input circuit of the detector through a transformer 2i). The mixing signal is of a large amplitude compared with the reflected signal and is applied directly from the oscillator 9 overa line Iii-tothe input circuit ofthe detector l through; atransformer 2 I. Transformers 20 and 2l may be tuned to the mean frequency of the carrier wave of the: reflected signal and of the mixing signal, respectively, by means of capacitors 22"and'Zconnectz..Y ed across the transformer secondaries.

with the detector l5 whereby the reflected-signal? and Vthe mixing signal are applied. iso-the. detector. in additive relation.

The outputs of diodes i6 and Il iiowithrougriV a. common output resistor'23 which is connected in series with the detector l5. andthe transformer secondariesv and which is: shunted by,Y a.- carrierfrequency bypass capacitor, 3i). Int-he circuit as illustrated, a capacitor Zitieconnectedirrseries with the detector l5. and outputresistor 23 to apply a balancingbiasto the diodes I Biand tzLas explained below, which ensures.- equal directeur.- rent in the diodes. This-alsoensures equal-.alterna-ting current through thediodes lsand IPL-since a diode-s A.C. characteristic isr similarv to its- D.-C. characteristic. If. the two: diodes-are reasonably well matched, the desired balance. is thusobtainedautomatically without requiring any adjustments, anda anyamplitude modulation that may be present isbalanced out. in the. output re` sistor 23 since the amplitude. modulation. outputsof the two:v diodesare. in. phase opposition..

It Will" be noted that the secondaries.` oiztransformersrfZ and 2l are in series with each: other andri'n series? The desired-beat' frequency signaL will appear in the detector output..however,l as explained later in. connection withv Figs; 2 and-2a,- and-.it is suppliedthrough-anaudio amplifier. 35- to iabeatfrequency indicator or. counter 36;

The action of capacitor 24" in providingautomaticV balancing ofthe circuit isas-folloWss If 4the current ilovw foragiven. applied voltagef is the#v same through diode IB as-through: diode l;1,.t-hen thecharge received. bycapaciton- 24. due to.: the positive half. cyclesZ or R.E. current flowing through diode- I5I will becompletelyremoved byV an equal current owthrong-l1y diode I6.- dur-ing the negative halfy cycles.. However, assume for example. that with. a. given. applied voltagethe currentflow through: diode: lfd-isf greater. than that through diode Il. Then-achargewillfbuild; up on capacitorl .213? and'.- the side.of capacitorj 2li-` connected to the cathode ofsdiode Iii/and totheY plate of-diode Il. will.- be positiva. Thusfcurrent iiow through diode Hwill lbe reduced byf-the biasand that through diode Il' will be increased until. an equilibrium condition is reached Where the currentsi'low is thesame. inf.thetwo.-diodes-.

Vl/fl'iiletheA capacitory 2l.-causestheV circuit to balance-automatically and-is desirable-for this-rea-` sonit may be-omitted'and-the amplitude-modulation will stillbe balancedr out satisfactorily provided` theA diodes; If and Il areY closely matched.' However, if capacitor-24-is omitted. it usually is desirableto provide an adjustable bi.- asing. sourcesuch as= abatteryv 251- shunted` byI a potentiometer 28.` 'Ihefpolarityn ofbattery 2-1 de pendsupon whichY one of the diodestends;tofpassr the. more currentat a-given-` appliedvoltage. A switch- 29 f connects thetdiodes: either to. capacitor 24 or to the battery 21.

The manner in which the desired beat frequency is produced will be seen by referring to Figs 2 and 2a. When the received signal A and the mixing signal B have the phase relation shown in Fig. 2, the negative half cycle of the added signal A-I-B is flattened and the rectified output ilowing through output resistor 23 will have the Wave shape shown in Fig. 2a. It is evidentv that the currentshown in Fig. 2a has a D.C. component. This component reverses at f the beat frequency as the relative phase of the received signal A and the mixing signal B changes;v and the resulting audio beat signal is suppliedv to the indicator 36.

It should be understood that the invention is nOtJimiteditothe particular circuit shown in Fig. 1'; For example, the detector l5 may be replaced by a'vacuum tube having one or more grids as-described in the above-mentioned Carlson patent, but' its. characteristic curve should, like the detector I5 be substantially symmetrical with respect to the A.-C. axis of the applied signals.

Fig. Bshows another type of. detector thatmay be used in place ofthe-double diode detector of Fig.. 1.. It comprises a Vacuum tube 3| having two indirectly heated cathodes 32 andf33 across which the voltage A-l-Bis. applied. The direction of current iiow. throughthe-tubel depends upon the polarity ofthe appliedvoltage; The voltage vs. currentcharacteristic of tube 3.| is similar tothe-characteristicshowninFig. 2 for detector i5. The operation with the detector. 3l is the same as that described in connectionwith Fig. 1.

Fromthe foregoing it willbe apparentthatour improved circuit has. the` important. advantage that nOcriticaLadjustments arefrequired tobalance outv any amplitudemodulation that may be present. If the capacitor. ill-is utilized, the condition of balance. is reached'` automatically. If, instead, the biasing source Zl-isutilized, the circuit mayy be adjusted. tobalance by. the adjust-` mentof a single. bias voltage value. Another adv vantage of the presentinvention .is .thatone side of thecircuitmay be. grounded, thusfavoiding. the diiculty of. obtaining an. exact balance with respect to agroundedcenter. point requireddn some circuits.

Weclaim as our. invention.:

1.. A.. distance determining system comprising meansA for transmitting to a reflecting object a radio carrier. wave which is.y cyclically' frequency modulated, a detector, meansfor receiving the signal reflected .from said` object and.for. applying itto. said detector, .means forf also applying to said detector a. mixing.: signal that is obtaineddirectly from said transmitting means and. whichhas aV frequency equal-.to one-half the frequency of the frequency modulated wave: that is transmitted to said objectwhereby the detector output-includes a beatl frequency signal-which is a measure of the ing. it to said detector;.means forA alsoapplying to said: detector a mixing: signalthat. is ,obtained irectly from said transmitting means and which has a frequency equal to one-half the frequency of the frequency modulated wave that is transmitted to said object whereby the detector output includes a beat frequency signal which is a measure of the distance from said transmitting means to said object and whereby any amplitude modulation that may be present is substantially balanced out, and a capacitor connected in series with the detector and the output impedance element whereby a bias voltage will appear across said capacitor to correct any unbalance due to lack of symmetry of the detector characteristic about the alternating current axis of the applied signals.

4. A distance determining system comprising means for transmitting to a reflecting object a radio carrier wave which is cyclically frequency modulated, a detector comprising means for conducting direct current in either direction nonlinearly with respect to the applied voltage and with the current flow in the two directions symmetrical with respect to the A.C. axis of the applied voltage, an output impedance element for said detector, means for receiving the signal reflected from said object and for applying it to the input of said detector, means for also applying to the input of said detector a mixing signal that is obtained directly from said transmitting means and which has a frequency equal to one-half the frequency of the frequency modulated wave that is transmitted to said object whereby the detector output appearing across said output impedance element includes a beat frequency signal that is a measure of the distance from said transmitting means to said obj ect and whereby any amplitude modulation that may be present is substantially balanced out.

5. The invention according to claim 4 wherein a capacitor is included in series with the detector and the output impedance element whereby a bias voltage will appear across said capacitor to correct for any unbalance due to any lack of symmetry of the detector characteristic with respect to said A.C. axis.

6. A distance determining system comprising means for transmitting to a reflecting object a radio carrier wave which is cyclically frequency modulated, a detector, an output impedance element for said detector, means for receiving the signal reflected from said object and for applying it to said detector, means for also applying to said detector a mixing signal that is obtained directly from said transmitting means and which has a frequency equal to one-half the frequency of the frequency modulated wave that is transmitted to said object whereby the detector output includes a beat frequency signal which is a measure of the distance from said transmitting means to said object and whereby any amplitude modulation that may be present is substantially balanced out, said detector comprising a pair of diodes connected in parallel with each other and in series with said output impedance element and said receiving means, said diodes being connected for current conduction in opposite directions.

7. The invention according to claim 4 wherein a capacitor is included in series with the detector and the output impedance element whereby a bias voltage will appear across said capacitor to correct any unbalance due to a difference between the characteristics of the two diodes.

8. A distance determining system comprising means for producing a radio carrier wave which is cyclically frequency modulated, means for doubling the frequency of said modulated wave and for then transmitting it to a reflecting object, a detector, means for receiving the signal reflected from said object and for applying it to said detector, means for also applying to said detector as a mixing signal a portion of said modulated carrier wave before its frequency is doubled whereby the detector output includes a beat frequency signal that is a measure of the distance from said transmitting means to said object and whereby any amplitude modulation that may be present is substantially balanced out.

9. The invention according to claim 6 wherein the detector characteristic is substantially symmetrical with respect to the alternating current axis of the applied signal.

10. A distance determining system comprising means for producing a radio carrier wave which is cyclically frequency modulated, means for doubling the frequency of said modulated Wave and for then transmitting it to a reflecting object, a detector comprising means for conducting direct current in either direction non-linearly with respect to the applied voltage and with the current flow in the two directions symmetrical with respect to the A.C. axis of the applied voltage, an output impedance element for said detector, means for receiving the signal reflected from said object and for applying it to said detector, means for also applying to said detector as a mixing signal a portion of said modulated carrier wave before its frequency is doubled whereby the detector output appearing across said output irnpedance element includes a beat frequency signal that is a measure of the distance from said transmitting means to said object and whereby any amplitude modulation that may be present is substantially balanced out.

11. The invention according to claim 10 wherein a capacitor is included in series with the detector and the output impedance element whereby a bias Voltage will appear across said capacitor to correct for any unbalance due to any lack of symmetry of the detector characteristic with respect to said A.C. axis.

12. In combination, means for transmitting to a reflecting object a radio carrier wave, a symmetrical non-linear detector, means for receiving the signal reflected from said object and for applying it to the input of said detector, means for also applying to the input of said detector a mixing signal that is obtained directly from saidY transmitting means and which has a frequency equal to one-half the frequency of the wave that is transmitted to said object whereby any amplitude modulation that may be present is substan tially balanced out.

WENDELL L. CARLSON. VERNON LANDON. 

